Prerequisites

In order to install and configure Drupal for Facebook, you need to do a couple things first.

Facebook Account

You are going to need a Facebook account. If you don’t have one, go ahead and create one now.

Devel Module

This module is not required on your production site but will be helpful on your development site. The FB module comes with the DFF (Drupal for Facebook) Devel module and provides blocks and messages that help when developing and debugging Apps. If you intend on using the DFF Devel module, download the devel module and enable it. (You wont get very far without this).

PHP JSON

Facebook requires your PHP installation to have JSON extensions to be installed. As of PHP 5.2.0, the JSON extension is bundled and compiled into PHP by default.
To check which version of PHP you are using for your D6 site go to admin > reports > status reports. If you have something less than 5.2, your PHP installation might have JSON extensions already installed. Please check with your hosting service provider. Information about JSON extensions is located at http://us.php.net/manual/en/json.requirements.php. Your hosting service might need this link. If you don’t have this installed, you will not be able to push content from your site to Facebook.